In recent years, it has become common practice, particularly in clinical oncology, to simultaneously administer several drugs to patients for therapeutic purposes. A vast literature, both experimental and clinical, has shown that the presence of one drug may markedly influence the intensity, duration, and pharmacologic effects of another drug absorption, in the binding of drugs to plasma proteins, which may directly or indirectly influence drug distribution, drug metabolism or drug clearance via the bile or urine. Many drugs and other foreign chemical compounds known collectively as xenobiotic are inactivated by enzymes present in liver microsomes, primarily by oxidation and conjugation. On the other hand some drugs and oncolytic agents are activated by these enzymes. Our major efforts are directed at understanding how drugs interact with each other and with biological organisms so that human drug therapy can be based on more rational grounds.